Saving a .dat file to Sd

Windows 10. Surface Book. I've downloaded a .dat file to update a camera's firmware. I copy the file to SD. When I take the SD out the file disappears. Please tell me what I'm doing wrong.

Where is the .dat file disappearing from, the camera or the computer? Most times reformatting a memory stick/card in a computer will prevent a camera from using it, best to reformat in the camera itself. My computers can read the SD/SDHC cards from the cameras without needing reformatting.

I formatted the SD card in camera as instructed. I inserted it into the computer and copied the .dat file to it. It's not seen by the camera nor is it there when I put it back into the computer. I checked by doing it all again. Saved to Sd. Took the SD out of the computer. Put the SD back into the computer and the .dat file is gone. Insanity looms. The .dat file disappears from the SD.

Could be be a Hidden file? Some programs do that to their own .dat files.

How are you removing the SD card? Are you using the "Safely Remove Hardware" icon in the task tray? Or are you right clicking on the Drive letter and choosing "Eject" before removing it?

If you are not doing this, then the file may not have been fully written to disk when you take it out. Ejecting or Safely Removing will flush all buffers and make sure it's completely written to disk.